The present invention relates to a telecommunications system, and more particularly to a frequency measurement isochronous clocking transmission method and apparatus.
In data communications, it is extremely important to maintain the same clock frequency between a transmitting and a receiving location. Often, a connection is desired in which the clock frequency is not related to that of the transmission system. Such a system is called an isochronous clock transmission system as the generated clock at the receiving end is the same frequency as at the transmitting end. Prior art approaches use a technique called pulse stuff synchronization or positive justification. This method uses special bits in the frame to adjust the clock frequency, and is well suited to circuit switched environments. Packet switched environments which desire isochronous clocking and do not have continuous packets must use another method. One such packet system is described in U.S. Pat. No. 4,771,425 (which is owned by the same assignee as the present invention), the details of which are hereby incorporated by reference.
Isochronous clock transmission systems are known in general. However, prior art isochronous clock transmission systems have an inherent need for continuous data transmission. This is not desirable or possible in most packet transmission systems. The prior art is known as "pulse-stuff synchronization" or "positive justification" and is used in many products. The DSIC telecommunications standard by Bell Communications Research describes the bits used in a typical application.
Many other products based on this system exist by companies such as Datatel and Timeplex. Pulse stuff synchronization functions by having specific bits in the data stream represent clocking information. These bits represent the difference between the actual clock frequency and the fractionally higher reference frequency. In the receiver, these bits are typically fed into a modulo n or n+1 divider in a phased locked loop to regenerate the input clock frequency. This type of system has an inherent need for continuous data transmission, as the bits are needed as input to the phased locked loop on a continual basis. An essential aspect of isochronous clock transmission systems is that they have 0 Hz long term frequency difference between the incoming and outgoing clocks. This need makes pulse stuff synchronization in such systems where the data is not always available, such as packet systems, untenable. A recent patent issued to a pulse stuff synchronization system is U.S. Pat. No. 4,675,886 for a frame synchronization device.